It means that one of the values in x or y is 0, or negative, or has a fractional component and is not exactly an integral value.
Note that if you computed x or y, then you might have suffered from round-off. See http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F